Wr. Leaitch, OBSERVATIONS PERTAINING TO THE EFFECT OF CHEMICAL TRANSFORMATION IN-CLOUD ON THE ANTHROPOGENIC AEROSOL-SIZE DISTRIBUTION, Aerosol science and technology, 25(2), 1996, pp. 157-173
Airborne observations from two case studies of continental cloud, one
of summertime cumulus and one of autumn stratus, are examined for evid
ence of changes in the size of the accumulation-mode aerosol particles
due to S(IV) oxidation in cloud. Such changes might affect the effici
ency with which the aerosol scatters light and its ability to function
as cloud condensation nuclei (CCN), and hence impact climate. In both
cases, there were strong anthropogenic contributions to the aerosol,
and the measured particles (0.17-2 mu m in diameter) were not scavenge
d with 100% efficiency: the relative number concentrations of measured
particles activated in cloud were about 70% for the cumulus and 15%-5
0% for the stratus. Within the measurement resolution, the apparent ef
fect of cloud processing in the cumulus case was to nearly preserve th
e shape of the size distribution of the measured aerosol, even after t
he mass addition of sulfate by S(IV) oxidation. This can be accomplish
ed through the activation of sufficiently small particles to balance t
he increase in larger particles. In the stratus case, the aerosol size
distributions associated with the cloud are bimodal with peaks at 0.2
3 and 0.35 mu m. The second peak coincides closely with the lower size
of particles activated in the cloud suggesting that S(IV) oxidation o
r possibly coalescence scavenging contributed to its formation.